BAFF receptor and TACI in B-1b cell maintenance and antibacterial responses

Although evidence of the protective immunity conferred by B-1b cells (CD19⁺B220⁺IgM^hiMac1⁺CD5^-) has been established, the mechanisms governing the maintenance and activation of B-1b cells following pathogen encounter remain unclear. B cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) mediate their function in mature B cells through the BAFF receptor (BAFFR) and transmembrane activator and CAML interactor (TACI). BAFFR-deficient mice have lower numbers of B-1b cells, and this reduction is directly proportional to BAFFR levels. The generation of B-1b cells is also dependent on the strength of B cell receptor (BCR) signaling. Mice with impaired BCR signaling, such as X-linked immunodeficient (xid) mice, have B-1b cell deficiency, indicating that both BCR- and BAFFR-mediated signaling are critical for B-1b cell homeostasis. Borrelia hermsii induces expansion and persistence of B-1b cells in xid mice, and these B-1b cells provide a heightened protective response. Toll-like receptor (TLR)-mediated stimulation of xid B cells results in a significant increase in TACI expression and restoration of TACI-mediated functions. The activation of TLR signaling by B. hermsii and BCR/TLR costimulation-mediated upregulation of BAFFR and TACI on B-1b cells suggests that B-1b cell maintenance and function following bacterial exposure may depend on BAFFR- and TACI-mediated signaling. In fact, the loss of both BAFFR and TACI results in a greater impairment in anti-B. hermsii responses compared to deficiency of BAFFR or TACI alone.